Wireless communication systems should provide for a large number of secure (or private) communication channels within their allotted frequency space. In order to achieve these goals, spread spectrum systems have been developed. In a spread spectrum type system, spreading codes are used that allow multiple channels to occupy the same frequency range. In order to successfully demodulate a channel, the spreading code and covering code used in connection with the channel must be known. When a demodulation processor is tracking a particular single path, signal paths associated with other transmitters appear to that processor as noise.
In order to provide for reliable communications, spread spectrum systems typically track multiple signal paths in connection with establishing and maintaining a communication channel between a pair of end points. The different signal paths may result from redundant signals that are provided by additional base stations and base station sectors, or from reflected or multipath versions of signals. In a typical receiver, a number (e.g., 4-6) demodulation processors or fingers are provided, and each of these fingers is assigned to track a different signal path. In order to obtain information regarding the different signal paths that are available to a receiver, a searcher demodulation processor or finger is provided. In a typical receiver, the searcher finger detects and identifies signals by pseudo-random number (PN) code offsets and signal strength. Because signal paths other than the signal paths being tracked appear as noise to a demodulation processor, the signal to noise ratio with respect to a tracked or desired signal path can be low, which can result in a communication channel with poor quality and reliability. In particular, signals from sources that are in close proximity to the receiver can drown out signals from sources that are farther away from the receiver. Accordingly, because of this “near-far” problem, signal diversity is limited. In addition to leaving communication channels more vulnerable to interruption, relatively weak signals that might otherwise be available to a receiver may lie beneath the noise floor created by other relatively strong signals.
The encoded channels broadcast by a base station generally do not interfere with one another due to the orthogonality of the covering Walsh codes or the quasi-orthogonality of the covering quasi-orthogonal functions (QOF). However, a spread spectrum communication system, such as a direct sequence, code division multiple access (DS-CDMA) system is still subject to two forms of multiple access interference on the forward link. Co-channel interference consists of multipath copies of signal paths that are delayed in time with respect to a signal path of interest. Such signals can cause interference because the orthogonality of the Walsh covering codes is lost whenever a time offset exists between two codes. Specifically, when aligned, Walsh codes form an orthogonal basis, but there may be high cross correlations when they are not aligned. Cross channel interference occurs when a combination of transmissions from more than one base station sector or base station are received at the RF front end simultaneously. Each base station sector is distinguished by a unique PN short code offset. However, the sequence has minimal, but non-zero cross-correlation properties. This manifests itself as cross-correlation interference between signals originating from different base station sectors. As a result, a signal transmitted from another base station that is received at a much greater power level is capable of masking the signal of interest due to the non-zero cross-correlation of the short code and the unaligned Walsh codes.
Methods for removing interfering signal paths from received signal streams have been developed. For example, interference cancellation is a feature incorporated into many spread spectrum receivers designed for CDMA systems, such as the cdma2000 and W-CDMA standards for wireless communications. In particular, interference cancellation receivers are a class of spread spectrum communication system receivers that have the capability to remove or reduce the interference from an interfering source or sources. Most methods of interference cancellation require the estimation of channels in the interfering signal source. Accordingly, the identity of valid channels within a signal path and the amplitudes of those channels must be estimated. Other methods, which utilize projection based methods for interference calculation require the construction of an interference vector to represent the direction of the interference from active channels within an interfering signal source, but not necessarily the amplitude of those channels.
Spread spectrum communication systems were initially developed using symbols of a single length. For example, IS-95 systems use a 64 chip symbol length. However, more recent communication standards, such as cdma2000, have introduced shorter length codes (supplemental channels) that coexist with longer codes. For example, to facilitate high-speed data transmissions, communication systems that allow symbols that are as short as four chips long for data transmission, while at the same time allowing channels utilizing symbols that are 64 chips long or more for voice communications have been developed. The use of symbols of different lengths within a single communication system has complicated the task of identifying active channels in a signal path. In addition, because the use of shorter length codes invalidates certain families of longer length codes, existing methods for identifying active channels can be inefficient when applied to systems supporting the use of multiple symbol lengths.
The fast Walsh transform is a known method for efficiently calculating the amplitudes of multiple channels within a signal path. In particular, the fast Walsh transform method can be used to calculate the amplitudes of channels that are covered using an orthogonal family of codes known as Walsh codes. However, conventional methods for applying the fast Walsh transform process have not been capable of efficiently estimating channels in connection with communication systems that support multiple symbol lengths. In addition, conventional methods have been incapable of efficiently constructing an interference vector for interference cancellation.